1. Field of Invention.
This invention relates to an optical disk test system for measuring the recording characteristics of optical disks.
2. Description of the Prior Art.
Generally, the recording characteristics of optical disks depend upon whether the recorded information can be correctly reproduced, and are based upon the error rate. This error rate is not sufficient for grasping the content or tendency of the errors which have occurred in the optical disks. These measurement results cannot be exploited for quality controls in the manufacturing process of the optical disks. Thus, in the optical disk test system, the recording characteristics of the optical disks have to be divided into several items for measurements. More specifically, the magnitudes of signals can be grasped by measuring the reflectivity. The optimum quantity of light for writing and reading and the qualities signals can be caught with the carrier/noise ratio (referred to as CNR)and their details can be grasped through crosstalk and waveform analysis.
On the other hand, the transparency is utilized for evaluating the optical absorption and lifetime of a recording film. However, if the CNR is more than 45 dB, the main cause of errors is flaws (i.e. defects). The length or size of defects and their distribution are useful measures for evaluating the burst errors and for improving the production process. Hence, the measurements of defects are effective for evaluating the recording characteristic of the optical disks, and the measurements of reflectivity relating thereto are the center of the test system. The presence or absence of defects are detected in terms of changes in the quantity of reflected light on the recording film surface.
FIG. 1 is a diagram depicting a conventional reflectivity measuring system comprising an optical disk 1, a recording film 11 of the disk 1, a laser beam source 31, a half mirror HMR1, and a light receiving element 37. Optical disk 1 is irradiated with a parallel beam having a diameter of about 1 mm. The reflected beam is received by light receiving element 37 so that the reflectivity of disk 1 is determined in terms of the intensity of the reflected beam.
In the reflectivity measuring system based on this principle, however, the reflected beams from film 11 and the surface of disk 1 interfere so that only the former reflected beam cannot be accurately measured. Since the parallel beam is used for measurements, on the other hand, the measured values will change greatly even in case the angle of incidence on optical disk 1 changes only slightly. Because the beam has a large diameter, moreover, minute defects cannot be finely detected. Since the parallel beam has a state different from that for practical reproduction, it is impossible to determine reflectivity conforming to practical reproduction. In addition, the system under consideration has an optical system which is different in construction from those used for measuring the CNR and cross talk so that a plurality of measuring systems are required for evaluating a series of different recording characteristics.